The invention relates to a circuit arrangement for infrared detectors, and more especially for a thermal viewing device with photoresistors as detector elements.
Highly sensitive devices for detecting infrared radiation, more especially thermal viewers, have a detector which consists of a plurality of independent detector elements. In the present state of the art in the wave length range around 10 microns the maximum number of detector elements is limited to approximately 300. For this reason the image has to be optically scanned. In the wave length range around 4 microns there are admittedly detectors with 250,000 detector elements. The quantum efficiency of these detectors is however so poor that for highly sensitive devices in this range as well the image is scanned with a substantially smaller number of detector elements.
In the present state of development CCD detectors for the above-mentioned infrared range operating on the electronic scan principle are not available. Therefore each detector element is provided with a input amplifier and an output amplifier.
Furthermore in these wave length ranges the technology of the production of detector zones with photodiodes have not been fully developed. For this reason photoresistors are used for the detector elements. The amplifiers thus have to be optimized for photoresistors as signal sources.
Owing to the large number of channels, for instance 288, in current technology a large number of amplifier systems are involved: 288 input amplifiers and 288 output amplifiers are required. A further point is that the amplifiers are capacitively coupled so that a large number of coupling capacitors are required as well.
The capacity of the coupling capacitors is comparatively large since the scan frequency only amounts to 50 Hz. For this reason a compromise is often made and the coupling capacitors are not made large enough, this leading to undesired pulse tilts in the signal.
A still further disadvantage in the known devices is that the 288 channels are only combined by a multiplexer to one channel after the output amplifier. At this point of combination the signal level is already comparatively high. This means that the setting means for brightness and contrast have to be arranged on the input side of the multiplexer in order to obtain the necessary dynamic range of signal transmission. Accordingly it is necessary to have at least 576 setting members which have to be in synchronism with each other. In the present state of development however, such synchronism is 30 difficult to achieve.